M&R 8.1/8.2 Drugs & receptors

Question 1

Concentration of drug molecules around a receptor is critical in determining drug action. Which has the same number of molecules - drugs at equivalent molar concentration or drugs at equivalent concentration by weight?

Answer 1

Drugs at equivalent molar concentration

Question 2

What is the equation for molarity (M)?

Answer 2

Molarity (M) = g/l / molecular weight

Question 3

Define mM, uM, nM and pM in moles (M)

Answer 3

Millimolar (mM) = 10^-3M
Micromolar (uM) = 10^-6M
Nanomolar (nM) = 10^-9M
Picomolar (pM) = 10^-12M

Question 4

1 mole of a substance contains how many molecules?

Answer 4

6x10^23 molecules (Avogadro’s constant)

Question 5

If substances have different molecular weights but the same molarity, do they have the same concentration of molecules?

Answer 5

Yes - they will both have Avogadro’s constant of molecules per mole

Question 6

What are antagonists?

Answer 6

ligands that have affinity but no efficacy

They do not activate the receptor but prevent the binding of an agonist

Question 7

What are agonists?

Answer 7

Ligands that have affinity and intrinsic efficacy

They can activate the receptor

Question 8

What is intrinsic efficacy vs efficacy?

Answer 8

Intrinsic efficacy is the ability of a ligand to activate a receptor

Efficacy is the ability of a ligand to cause a measurable response (it depends on both intrinsic efficacy [how good it is at activating the receptor] and cell/tissue dependent factors

Question 9

Do anatgonists have intrinsic efficacy?

Answer 9

No, they have affinity but no efficacy

Question 10

What method can be used to measure binding of ligands to receptors?

Answer 10

By binding a radiolabelled ligand (radioligand)
Next separate the free radioligand from bound via centrifugation
Then measure the amount of (bound) radioligand left
Low [ligand] = low binding, high [ligand] = high binding

Question 11

How does ligand concentration relate to receptor binding?

Answer 11

The higher the concentration of ligand, the more binding to its receptor

Question 12

What shape is a binding curve of drug concentration (x) against proportion of bound receptors (y)

Answer 12

A rectangular hyperbola

Question 13

What is KD/Kd? What does it give information about?

Answer 13

Kd = the dissociation constant
= the concentration of ligand required to bind 50% of the available receptors
It is a measure of affinity (lower value=higher affinity)

Question 14

What is Bmax? What does it give information about?

Answer 14

Bmax= the maximum binding capacity
= the concentration of ligand required to occupy ALL of the receptors
Therefore it gives information about the number of receptors (if we know how many molecules of ligand it is we can calculate the number of receptors)

Question 15

What shape is a binding curve of log[drug] (x) against proportion of bound receptors (y)

Answer 15

Sigmoidal

Question 16

When should a concentration-response curve be used vs a dose-response curve?

Answer 16

Concentration-response curve is used when measuring response in cells/tissues

Dose-response curve is used when measuring in a whole animal (and we don’t know how much is working at the site of action)

Question 17

What shape is a dose or concentration -response curve plotting % response (y) and log[drug] (x)?

Answer 17

Sigmoidal

Question 18

What is Emax?

Answer 18

(On a concentration-response curve)

Emax = effect max (total max effect of that drug)
= the amount of drug causing a response of 100% of the maximum

Question 19

What is EC50?

Answer 19

(On a conc-response curve)
EC50 = the effective concentration giving 50% of the maximal response of that drug
= a measure of agonist POTENCY (depends on both affinity and efficacy)

Question 20

What is the concentration giving 50% of the maximum inhibition, for an inhibitory drug?

Answer 20

IC50

Question 21

What is potency?

Answer 21

affinity and efficacy = potency

Question 22

What is efficacy?

Answer 22

The ability of an agonist to cause a measurable response

A combination of intrinsic efficacy and cell/tissue dependent factors

Question 23

What is the relationship of Emax values of agonists to efficacy?

Answer 23

Agonists with different Emax values have different efficacy

But agonists with the same Emax may not have identical efficacy (one may have less efficacy but more affinity and the other have more efficacy but less affinity, making the response the same)

Question 24

If two drugs with different efficacies are plotted on the same response curve, which drug will be further left?

Answer 24

The drug with HIGHER efficacy will be further to the left, because it requiring a lower concentration of drug in order to reach 50% of maximal response

Question 25

Salbutamol has a higher affinity for beta1 than beta2 receptors. How does manage to selectively affect beta2 receptors in asthma?

Answer 25

It has higher EFFICACY at beta2 receptors than beta1 receptors

The route of administration (inhaler) targets it to the lungs

Question 26

How does salmeterol selectively affect beta2 receptors in asthma, even though it has equal efficacy at beta1 and beta2?

Answer 26

It has much greater AFFINITY for the beta2 receptor

So while it is equally good at activating beta1 and beta2, it is much better at binding to beta2

Question 27

Out of affinity, intrinsic efficacy, efficacy and potency, which are variable and which are fixed?

Answer 27

Affinity is fixed
Intrinsic efficacy is fixed
Efficacy is variable (because of cell/tissue dependent factors)
Potency is variable (because efficacy is variable)

Question 28

Explain the general concept of spare receptors

Answer 28

Relates to cell/tissue dependent factors that influence efficacy (and therefore potency)
Often a response is limited by other factors (e.g. a muscle can only secrete so much) so occupying more receptors cannot generate a bigger response
Therefore not all the receptors are contributing to the biological response - some are ‘spare’

Question 29

What do we see on the response curve due to spare receptors?

Answer 29

Less than 100% occupancy leading to 100% response

Question 30

In what circumstances are spare receptors often seen? Why?

Answer 30

When receptors are catalytically active (e.g. GPCRs or tyrosine kinases) - Because of amplification in the signal transduction pathway

Or when response is limited by a post-receptor event (e.g. contraction)

Question 31

What is the advantage of having spare receptors? Explain.

Answer 31

They increase SENSITIVITY - by allowing higher responses at lower conc of agonist

If no spare receptors then full response would require 100% occupancy - would need more than Kd of drug. If full response requires e.g. only 50% occupancy, then only need Kd of drug

Question 32

What effect does changing receptor number have?

Answer 32

Changes agonist potency, and can affect the maximal response.
E.g. if there are lots more receptors, may only need 10% occupancy to get full response. If there are much fewer receptors, 100% occupancy may not give full response (if insufficient receptor number)

Question 33

When do receptor numbers tend to increase?

What disadvantage can this have?

Answer 33

With low activity (up-regulated)

Can contribute to supersensitivity

Question 34

When do receptor numbers tend to decrease?

What disadvantage can this have?

Answer 34

Tend to decrease with high activity (down-regulation)

(for drugs) can contribute to tachyphylaxis/tolerance

Question 35

What is the name for an agonist that cannot produce a maximal effect, even with full receptor occupancy?

Answer 35

A partial agonist

Question 36

Define ‘partial agonist’

Answer 36

An agonist that cannot produce a maximal effect, even with full receptor occupancy

Question 37

For partial agonists, what is the relationship between EC50 and Kd?

Answer 37

EC50 ~= Kd

Question 38

What does partial agonist lack, compared to a full agonist?

Answer 38

Intrinsic efficacy

Question 39

Can a partial agonist be more potent than a full agonist?

Answer 39

Yes - because potency depends on AFFINITY as well as efficacy

Question 40

What are 3 advantages of partial agonists?

Answer 40

1. They can allow a more controlled response
2. Then can work in absence/low levels of endogenous ligand
3. If high levels of full agonist, can act as an antagonist

Question 41

What is the affinity and efficacy of buprenorphine like, with respect to morphine? Why is this useful?

Answer 41

Buprenorphine is a partial agonist at the same receptor (u-opioid) as morphine
It has a higher affinity than morphine (lower Kd)
But has a lower efficacy (can’t produce the full response)
Can be helpful for providing adequate pain control with less respiratory depression

Question 42

If a heroin addict accidentally injected buprenorphine instead, what would happen? Why?

Answer 42

Withdrawal symptoms - because the buprenorphine (partial agonist) has a higher affinity but lower efficacy than the full agonist so it acts as an antagonist of heroin. It is occupying the receptors but unable to generate a maximal response

Question 43

Under what circumstances may a partial agonist act as a full agonist? Why?

Answer 43

When receptor number is increased (using some ‘spare’ receptors)
It still has low intrinsic efficacy at each receptor, but there are sufficient receptors for it to generate a full response.

Question 44

Name 3 types of antagonism

Answer 44

1. Reversible, competitive antagonism
2. Irreversible, competitive antagonism
3. Non-competitive antagonism

Question 45

Describe some properties of reversible competitive antagonism

Answer 45

It depends on the dynamic equilibrium between ligands and receptors
Greater [antagonist] = greater inhibition
Surmountable (can be overcome by addition of more agonist)

Question 46

In terms of antagonists, what is IC50? What does it represent?

Answer 46

Concentration of antagonist giving 50% inhibition

Represents an index of antagonist potency

Determined by amount of antagonist but also amount if of agonist

Question 47

What do reversible competitive antagonists do to the agonis concentration response curve?

Answer 47

They cause a parallel shift to the right

i.e. more agonist is needed for the same response

Question 48

Give an example of a reversible competitive antagonist, where it works and what it is used for

Answer 48

Naloxone
High affinity RCA at u-opioid receptors
Can be used to revers opioid-mediated respiratory depression 9its high affinity means it competes effectively with other opioids (e.g. heroin) for receptors

Question 49

Describe some features of irreversible competitive antagonism

Answer 49

The antagonist associates with the receptor, but once bound it doesn’t really dissociate (dissociates either very slowly or not at all)
With increasing [antagonist] or increasing time, more and more receptors are blocked by antagonist
Non-surmountable

Question 50

What does an irreversible competitive antagonist do to the agonist concentration-response curve?

Answer 50

Initially causes a parallel shift to the right as spare receptors become filled by antagonist so more agonist is needed for the same response
Eventually suppresses the maximum response (‘squishes’ the curve downwards!) because antagonist bound to so many receptors that the agonist can’t bind to enough receptors for a full response (no matter how much agonist is added)

Question 51

Give an example of an irreversible competitive antagonist, the receptor it acts on and how it is used clinically

Answer 51

Phenoxybenzamine
ICA at alpha1-adrenoceptors
Used in hypertension episodes in phaeochromocytoma - blocks the action of the excessive adrenaline produced. Non-surmountable so once bound prevents even high concentrations of adrenaline binding

Question 52

What is the name of the site where an endogenous ligand usually bind to activate a receptor?

Answer 52

The orthosteric site

Question 53

What is the name of sites other than the orthosteric site, where the natural ligand doesn’t bind, but other molecules can bind & potentially alter what the receptor is doing?

Answer 53

Allosteric sites

Question 54

Describe the features of non-competitive antagonism

Answer 54

Molecules can bind to allosteric sites on a receptor and affect the affinity and/or efficacy of the receptor for the endogenous ligand at the orthosteric site

There is no competition for the binding site because they do not bind at the orthosteric site

Question 55

How does non-competitive antagonism affect the agonist-concentration curve? Which other type of antagonism is it similar to?

Answer 55

It has similar effects to irreversible competitive antagonism (suppresses the maximal response)
Tend to need additional experiments to distinguish the two

Question 56

What are some advantages of using non-competitive antagonists to regulate receptors at their allosteric sites?

Answer 56

1. Can give higher receptor subtype selectivity (allosteric sites tend to be more different, orthosteric sites similar across subtypes)
2. Often require orthosteric ligand to act - therefore can make drugs that are only active when receptor being stimulated by endogenous ligand (work in a more physiological way)
3. Can be small molecules (better for drugs) - orthosteric ligands often have to be large peptides

Question 57

Give an example of a non-competitive antagonist, where it acts and what effect this has

Answer 57

Cinacalcet
Positive allosteric modulator of the Ca2+ sensing receptor
Used in hyperparathyroidism to decrease extracellular Ca2+